Bash Scripting — File Existence and Permissions in Scripts

Chapter 18 — File Existence and Permissions in Scripts

Welcome to Chapter 18! 🎉 This is where your scripts start behaving like production-grade tools instead of fragile toys.

Every real-world script touches the filesystem — reading config files, writing logs, creating directories, setting permissions. If your script blindly assumes a file exists or a directory is writable, it will break in production. This chapter teaches you how to write scripts that check first and act safely.

💡 Remember from Part 2? We covered chmod, chown, and basic file permissions interactively on the command line. Now we’ll use those same concepts inside scripts to automate permission management.

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What You’ll Learn

#	Topic	What You’ll Master
1	File Test Operators	-e, -f, -d, -L, -r, -w, -x, -s, -nt, -ot, -ef
2	Defensive File Handling	Check-before-open, absolute paths, quoting filenames with spaces
3	chmod and chown in Scripts	Automate permissions — chmod, chown, stat, EUID checks
4	Checking for Root Privileges	$EUID vs $UID, root guard patterns, sudo re-execution
5	Directory Operations in Scripts	mkdir -p, safe cd, pushd/popd, mktemp -d
6	Practice Exercises	5 hands-on challenges from easy to advanced

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Prerequisites

Before starting this chapter, make sure you’re comfortable with:

• Chapter 15–17: Variables, conditionals (if/else), and loops
• Part 2 — File Permissions: chmod, chown, ls -l output format
• Basic test syntax: [[ condition ]] and if statements

If any of those feel rusty, take a quick refresher — this chapter builds directly on them.

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Why This Matters

Consider this innocent-looking script:

#!/usr/bin/env bash
set -euo pipefail

# ❌ FRAGILE — assumes everything exists
cp /etc/myapp/config.yml /tmp/backup/
chmod 600 /tmp/backup/config.yml

What could go wrong?

1. /etc/myapp/config.yml might not exist → cp fails
2. /tmp/backup/ directory might not exist → cp fails
3. The script might not have read permission on the source
4. chmod might fail if the user doesn’t own the file

A defensive version checks all of these before acting. That’s what this chapter teaches.

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Quick Reference — File Test Operators

Here’s a cheat sheet you’ll use throughout this chapter:

Operator	True When…
-e file	File exists (any type)
-f file	Regular file exists
-d file	Directory exists
-L file	Symbolic link exists
-r file	File is readable
-w file	File is writable
-x file	File is executable
-s file	File exists and is non-empty
f1 -nt f2	File f1 is newer than f2
f1 -ot f2	File f1 is older than f2
f1 -ef f2	f1 and f2 are the same inode (hardlinks)

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What’s Next?

Start with File Test Operators → to learn the building blocks, then work through each topic in order. Finish with the Practice Exercises to cement your skills.

Let’s make your scripts bulletproof! 🛡️